Optical interferometer

ABSTRACT

The present invention relates to an integrated optical waveguide interferometer including a sensing waveguide with a path of interaction (with the localised environment) having different optical lengths, and to a method for determining the absolute status thereof after the introduction into the localised environment of (or changes in) a stimulus of interest and to a method for determining the absolute calibration status thereof before the introduction into the localised environment of (or changes in) a stimulus of interest.

The present invention relates to an integrated optical waveguideinterferometer including a sensing waveguide with a path of interaction(with the localised environment) of variable optical length, and to aprocess for determining the absolute status thereof after theintroduction into the localised environment of (or changes in) astimulus of interest and to a method for determining the absolutecalibration status thereof before the introduction into the localisedenvironment of (or changes in) a stimulus of interest.

Optical interferometry is a well-established technique in the field ofsensing. It is intrinsic to optical interferometers that the response iscyclical and provides identical information every integer multiple of 2πradians in phase difference. In order to track differences in opticalpath length over ranges greater than 2π radians, a fringe countingmethod is generally required. However in the case of integrated opticalinterferometers, a large stimulus applied rapidly to the sensingwaveguide may cause the loss of information through miscounting. Thisphenomenon is known as aliasing and is described hereinafter in detailwith reference to FIG. 1.

In order to address aliasing, it is known to exploit two or morewavelengths to provide a difference signal that acts as a slowly varyingphase position marker. In the case of optical interferometers forchemical and biological sensing, refractive index dispersion in thechemical or biological materials may complicate such an analysis.

It will be appreciated that over its lifetime, an optical interferometerwill be exposed continuously to a changing localised environment and itsabsolute status will vary accordingly. For example, the level ofmoisture absorbed into the sensing waveguide will vary according toatmospheric conditions. In practice, this means that the opticalinterferometer will need to be calibrated on each occasion before it isused (typically with a calibrant gas). This allows measured changes ineffective refractive index to be related exclusively to the introductionof or changes in a stimulus of interest. The need to calibrate theinstrument prior to each use is a great inconvenience to the user andthe use of standard material calibrants is restrictive.

If the history of the phase differences are not known or are lost duringan interruption in monitoring the optical interferometer, the usercannot determine its absolute status in response to an analyte. Startingor resuming operation may lead to significant errors since signal loss(aliasing) may have occurred. Typically this means that once an opticalinterferometer is calibrated, it is not possible to switch it offwithout the calibration being lost.

WO-A-01/36947 (Farfield Sensors Limited) discloses the use of thermal orwavelength biassing to make absolute measurements of an opticalinterferometer and to address aliasing. This requires rathersophisticated ancillary components to be incorporated into the sensorassembly.

The present invention seeks to greatly reduce the risk of aliasing andeliminate the need for standard calibration prior to use by exploitingan optical interferometer having a sensing waveguide with a built-inreference system. More particularly, the present invention relates to anintegrated optical waveguide interferometer having on its sensingwaveguide a well characterised, variable optical length path ofinteraction.

Thus viewed from one aspect the present invention provides an integratedoptical waveguide interferometer capable of detecting the amount of (egconcentration of) or changes in a stimulus of interest comprising:

-   a sensing waveguide capable of exhibiting a measurable response to a    change in a localised environment caused by the introduction of or    changes in the stimulus of interest, said sensing waveguide having a    path of interaction of variable optical length.

The integrated optical waveguide interferometer of the invention allowsthe user to determine its absolute status (regardless of the arbitraryphase position) before or after the introduction of a stimulus ofinterest. It is also a straightforward matter to verify that theintegrated optical waveguide interferometer of the invention remains inits previously determined state after suspension of operation (eg apower failure) has prevented continuous monitoring of the phase shift.This substantially eliminates the risk of the output of the integratedoptical waveguide interferometer being subjected to aliasing.

The integrated optical waveguide interferometer may be generally of thetype disclosed in WO-A-98/22807 or WO-A-01/36945.

In a preferred embodiment, the integrated optical waveguideinterferometer further includes:

-   one or more sensing layers capable of inducing in the sensing    waveguide a measurable response to a change in the localised    environment caused by the introduction of or changes in a stimulus    of interest.

In this embodiment, the integrated optical waveguide interferometer isadvantageously adapted to optimise the evanescent component so as toinduce in the sensing waveguide a measurable response. The integratedoptical waveguide interferometer may comprise a plurality of separatesensing layers to enable changes at different localised environments tobe detected.

The (or each) sensing layer may be or include the stimulus of interest.

In a preferred integrated optical waveguide interferometer of theinvention, the sensing layer comprises an absorbent material (eg apolymeric material such as polymethylmethacrylate, polysiloxane,poly-4-vinylpyridine) or a bioactive material (eg containing antibodies,enzymes, DNA fragments, functional proteins or whole cells). Theabsorbent material may be capable of absorbing a gas, a liquid or avapour analyte containing a chemical stimulus of interest. The bioactivematerial may be appropriate for liquid or gas phase biosensing. Forexample, the sensing layer may comprise a porous silicon materialoptionally biofunctionalised with antibodies, enzymes, DNA fragments,functional proteins or whole cells. In an integrated optical waveguideinterferometer for use in biological or chemical sensing, theinteraction of the stimulus of interest with the sensing layer may be abinding interaction or absorbance or any other interaction.

To optimise its performance, the integrated optical waveguideinterferometer may further comprise an inactive waveguide in which thesensing layer is substantially incapable of inducing a measurableresponse to a change in the localised environment caused by theintroduction of or changes in the stimulus of interest. The inactivewaveguide is capable of acting as a reference layer. It is preferredthat the sensing waveguide and inactive waveguide have identicalproperties with the exception of the measurable response to the changein the localised environment caused by the introduction of or changes inthe stimulus of interest.

In a preferred integrated optical waveguide interferometer of theinvention, the sensing waveguide (and/or inactive waveguide) comprisessilicon nitride or (preferably) silicon oxynitride.

Preferably the sensing waveguide and any additional waveguide (such as areference waveguide) is a planar waveguide (ie a waveguide which permitslight propagation in any arbitrary direction within the plane),particularly preferably a slab waveguide.

In a preferred embodiment, the variation in optical length of the pathof interaction is sufficient to ensure a variation in phase changecaused by the introduction of or changes in a stimulus of interest of<2π.

By way of example, the path of interaction may be stepped. Preferablythe path of interaction is of dual optical length. Particularlypreferably the difference in dual optical length is sufficient to ensurea difference in phase change caused by the introduction of or changes ina stimulus of interest of <2π.

The optical length of the path of interaction (L′) is related to thegeometrical length of the path of interaction (L) by the relationship:L′=λL/n(where λ is the free space wavelength of electromagnetic radiation and nis the refractive index of the path of interaction).

Thus the variable optical length of the path of interaction may beprovided by a variation in its geometrical length. Preferably thevariation in geometrical length is sufficient to ensure a variation inphase change caused by the introduction of or changes in a stimulus ofinterest of <2π.

For example, the geometrical length of the path of interaction may bestepped. Preferably the path of interaction is of dual geometricallength. Particularly preferably the difference in dual geometricallength is sufficient to ensure a difference in phase change caused bythe introduction of or changes in a stimulus of interest of <2π.

The variation in the geometrical length of the path of interaction maybe continuous. For example, the path of interaction may have a gradient.

The variable optical length of the path of interaction may be providedby a variation in its refractive index. The refractive index may bevaried intrinsically or dimensionally.

The refractive index may be varied intrinsically by varying thecomposition of the material of the sensing waveguide. For example, thesensing waveguide may be composed of two or more discrete portions ofmaterial of differing (and well characterised) composition. Preferablythe sensing waveguide is of dual composition.

Preferably the refractive index is varied dimensionally (which can beadvantageously achieved very accurately through know fabricationtechniques). For example, the refractive index may be varieddimensionally by varying the thickness of the sensing waveguide.Preferably the sensing waveguide is of dual thickness.

In a preferred embodiment, the integrated optical waveguideinterferometer further comprises a capping layer adapted to define thepath of interaction of variable optical length. The capping layer istypically in contact with the surface of the sensing waveguide.

The capping layer may incorporate a window which bounds the localisedenvironment (eg above the part of the sensing waveguide with which thecapping layer is in contact). For example, the window may bound asensing layer such as an absorbent material or a bioactive material(described in greater detail above). The shape of the window may betailored to precisely define the variation in the optical length of thepath of interaction.

By incorporating a window, the capping layer defines a path ofinteraction of (at least) dual optical length in which a first part ofthe modal field interacts with the medium in the window and a secondpart of the modal field interacts with the medium of the capping layer.

The composition (and therefore the refractive index) of a capping layermay be precisely determined using standard material deposition methodsfamiliar in semiconductor processing (for example, silicon dioxidedeposition using PECVD). Photolithography may be used in defining thewindow and typically has a resolution of less than 1 micron. Thevariation in path length can thus be controlled to this level.

Suitable capping layers are described in copending UK patent applicationno. 0203581.4 filed on Feb. 15, 2002 by Farfield Sensors Limited. Thecapping layer typically has a thickness of 10 microns or less,preferably 5 microns or less and may be composed of silicon dioxide.

Preferably the integrated optical waveguide interferometer constitutes amulti-layered structure (eg a laminated waveguide structure). In apreferred embodiment, each of the plurality of layers in themulti-layered structure are built onto a substrate (eg of silicon)through known processes such as PECVD, LPCVD, etc. Such processes arehighly repeatable and lead to accurate manufacture. Intermediatetransparent layers may be added (eg silicon dioxide) if desired.Typically the multilayered structure is of thickness in the range 0.2-10microns. A layered structure advantageously permits layers to be inclose proximity (eg a sensing waveguide and an inactive (reference)waveguide may be in close proximity to one another so as to minimise thedeleterious effects of temperature and other environmental factors).Preferably the integrated optical waveguide interferometer comprises astack of transparent dielectric layers wherein layers are placed inclose proximity. Preferably each layer is fabricated to allow equalamounts of electromagnetic radiation to propagate by simultaneousexcitation of the guided modes in the structure. Particularlypreferably, the amount of electromagnetic radiation in the sensingwaveguide/inactive waveguide is equal.

Electromagnetic radiation generated from a conventional source may bepropagated into the integrated optical waveguide interferometer in anumber of ways. In a preferred embodiment, radiation is simply input viaan end face of the integrated optical waveguide interferometer (this issometimes described as “an end firing procedure”). Preferably theelectromagnetic radiation source provides incident electromagneticradiation having a wavelength falling within the optical range.Propagating means may be employed for substantially simultaneouslypropagating incident electromagnetic radiation into a plurality ofwaveguides. For example, one or more coupling gratings or mirrors may beused. A tapered end coupler rather than a coupling grating or mirror maybe used to propagate light into the lowermost waveguide.

The incident electromagnetic radiation may be oriented (eg planepolarised) as desired using an appropriate polarising means. Theincident electromagnetic radiation may be focussed if desired using alens or similar micro-focussing means.

The integrated optical waveguide interferometer may further comprise:means for intimately exposing at least a part of the sensing waveguide(or at least a part of the (or each) sensing layer) to the localisedenvironment (eg as described in WO-A-01/36945). For example, the meansfor intimately exposing at least a part of the sensing waveguide (or atleast a part of the (or each) sensing layer) to the localisedenvironment may be a part of a microstructure positionable on thesurface of and in intimate contact with the sensing waveguide. Themicrostructure may comprise means for intimately exposing at least apart of the sensing waveguide (or at least a part of the (or each)sensing layer) to the localised environment in the form of one or moremicrochannels and/or microchambers. For example, an analyte containingchemical stimuli may be fed through microchannels or chemical reactionsmay take place in an analyte located in a microchamber. An analytecontaining chemical stimuli may be fed into the microchannels bycapillary action or positively fed by an urging means. The means forintimately exposing at least a part of the sensing waveguide (or atleast a part of the (or each) sensing layer) to the localisedenvironment may be integrated onto the sensing waveguide or may beincluded in a cladding layer. For example, microchannels and/ormicrochambers may be etched into the cladding layer.

The means for intimately exposing at least a part of the sensingwaveguide (or at least a part of the (or each) sensing layer) to thelocalised environment may be adapted to induce chemical or biologicalchanges in a static analyte containing a chemical or biological stimulusof interest. In this sense, the system may be considered to be dynamic.Chemical or biological changes (eg reactions) may be induced in anyconventional manner such as by heat or radiation.

As a consequence of the introduction of or changes in a physical,biological and/or chemical stimulus of interest in the localisedenvironment (ie a change in the refractive index of material in thelocalised environment), changes in the transmission of electromagneticradiation down the sensing waveguide occur which may be measured (ie ameasurable response). Due to the variation in optical length of the pathof interaction, the measurable response varies. Thus by comparing thevarying measurable response the optical interferometer of the inventionmakes available a slowly varying phase position marker over many cyclesof phase change and by dispensing with the need for standard calibrationallows measurements of the absolute status of the integrated opticalwaveguide interferometer before or after the introduction of a stimulusof interest.

Viewed from a further aspect the present invention provides a processfor determining the absolute status of an integrated optical waveguideinterferometer, said process comprising:

-   (A) providing an integrated optical waveguide interferometer as    hereinbefore defined;-   (B) irradiating the integrated optical waveguide interferometer with    electromagnetic radiation;-   (C) introducing to the localised environment a stimulus of interest;-   (D) measuring the variation in phase shift of the modal field    interacting with the path of interaction; and-   (E) calculating from the variation in phase shift the absolute    status of the integrated optical waveguide interferometer.

Preferably the variation in phase shift caused by the introduction of orchanges in a stimulus of interest is <2π.

In a preferred embodiment, the process of the invention comprises:

-   (A1) providing an integrated optical waveguide interferometer as    hereinbefore defined, wherein the path of interaction is of dual    optical length;-   (B) irradiating the integrated optical waveguide interferometer with    electromagnetic radiation;-   (C) introducing to the localised environment a stimulus of interest;-   (D1) measuring the difference in phase shift of the first and second    parts of the modal field interacting with the path of interaction of    dual optical length respectively; and-   (E1) calculating from the difference in phase shift the absolute    status of the integrated optical waveguide interferometer.

Preferably the difference in phase shift caused by the introduction ofor changes in a stimulus of interest is <2π.

Preferably the process further comprises:

-   (F) relating the absolute status to the amount (eg concentration) of    or changes in the chemical stimulus of interest. Methods for    performing this calculation will be familiar to those skilled in the    art.

Viewed from a yet further aspect the present invention provides a methodfor determining the absolute calibration status of an integrated opticalwaveguide interferometer, said process comprising:

-   (A) providing an integrated optical waveguide interferometer as    hereinbefore defined;-   (B) irradiating the integrated optical waveguide interferometer with    electromagnetic radiation;-   (C) measuring the variation in phase position of the modal field    interacting with the path of interaction; and-   (D) calculating from the variation in phase position the absolute    calibration status of the integrated optical waveguide    interferometer.

In a preferred embodiment, the method of the invention comprises:

-   (A1) providing an integrated optical waveguide interferometer as    hereinbefore defined, wherein the path of interaction is of dual    optical length;-   (B) irradiating the integrated optical waveguide interferometer with    electromagnetic radiation;-   (C1) measuring the difference in phase position of the first and    second parts of the modal field interacting with the path of    interaction of dual optical length respectively; and-   (D1) calculating from the difference in phase position the absolute    status of the integrated optical waveguide interferomete

In a preferred embodiment, steps (A) to (C) are performed at start-up(eg automatically).

An interference pattern may be generated when the electromagneticradiation from the integrated optical waveguide interferometer iscoupled into free space and the pattern may be recorded in aconventional manner (see for example WO-A-98/22807). A measurableresponse of the sensing waveguide to a change in the localisedenvironment manifests itself as movement of the fringes in theinterference pattern. The phase shift of the radiation in the sensingwaveguide may be straightforwardly calculated from the movement in thefringes.

Movement in the interference fringes may be measured either using asingle detector which measures changes in the electromagnetic radiationintensity or a plurality of such detectors which monitor the changeoccurring in a number of fringes or the entire interference pattern. Theone or more detectors may comprise one or more photodetectors. Wheremore than one photodetector is used this may be arranged in an array.

Preferably the integrated optical waveguide interferometer is adapted todetermine its absolute calibration status at start-up. Particularlypreferably the integrated optical waveguide interferometer is adapted todetermine its absolute calibration status automatically at start-up.

The present invention will now be described in a non-limitative sensewith reference to the accompanying Figures in which:

FIG. 1 illustrates in side view an integrated optical waveguideinterferometer of the prior art;

FIG. 2 illustrates in exposed plan view the an embodiment of theintegrated optical waveguide interferometer of the invention;

FIG. 3 illustrates the interference pattern produced from the left andright hand modal fields of the embodiment of FIG. 2;

FIG. 4 illustrates the two parallel modal fields φ_(l) and φ_(r) andtheir difference in path length of interaction;

FIG. 5 illustrates in exposed plan view an embodiment of the integratedoptical waveguide interferometer of the invention;

FIG. 6 illustrates an embodiment of the integrated optical waveguideinterferometer of the invention; and

FIG. 7 illustrates an embodiment of the integrated optical waveguideinterferometer of the invention.

FIG. 1 illustrates in side view an integrated optical waveguideinterferometer of the prior art designated generally by referencenumeral 1. The integrated optical waveguide interferometer 1 is of thetype described in WO-A-98/22807 which in this embodiment comprises asilicon substrate layer 4 a, silicon dioxide layers 4 b and 4 d, siliconoxynitride layers 4 c and 4 e and an absorbent sensing layer 4 f. Thesilicon oxynitride layer 4 c acts as the reference waveguide and thesilicon oxynitride layer 4 e acts as the sensing waveguide so that theintegrated optical waveguide interferometer 1 is deployed in evanescentmode. The assembly further comprises a photodetector 2 for detectingfringes 10 of an interference pattern 11 in the far field, a focussinglens 3 and a source (not shown) of electromagnetic radiation E.

The silicon oxynitride layer 4 e (the sensing waveguide) acts throughits evanescent field to provide a transduction mechanism for changes inthe optogeometrical properties of the absorbent sensing layer 4 f. Thepropagation constant of the sensing waveguide mode (β) is altered as theoptogeometrical properties of the absorbent sensing layer 4 f change andwhen the far field output of the sensing waveguide mode is recombinedwith that of the reference waveguide 4 c, the intensity distribution ofthe interference pattern 11 shifts.

The observed phase shift (Δφ) is related to the shift in relative phaseposition between the interfering modal fields of the sensing waveguide 4e and reference waveguide 4 c at the output face 5. Since the modalfield of the reference waveguide 4 c is essentially unaffected bychanges in the optogeometrical properties of the absorbent sensing layer4 f, the observed phase shift is almost exclusively due to changes inthe propagation constant of the sensing waveguide mode. Over a length(L) of a path of interaction (the length over which the evanescent fieldcan act as a transducer) the observed phase shift is given by:Δφ=ΔβL(where Δβ is the change in the propagation constant β per unit length ofinteraction). The interference pattern 11 produced in the far field isreproduced identically when:Δφ=n2π(where n is an integer).

Thus if a method of ‘fringe counting’ (ie the counting of n) is notemployed or the method cannot record rapid and large changes in phasequickly enough, it will be impossible to unambiguously determine thevalue of Δφ.

Absolute Measurements and Prevention of Aliasing

FIG. 2 illustrates in exposed plan view an embodiment of the integratedoptical waveguide interferometer of the invention. It is largely thesame as that described above with reference to FIG. 1 but the absorbentsensing layer 4 f has been replaced by a capping layer 4 g of knownrefractive index into which a window 15 of precisely defined dimensionshas been introduced by selective removal of the layer thereby exposingthe surface of the sensing waveguide 4 e. The window 15 provides a pathof interaction where the evanescent field of the sensing waveguide modecan interact with the medium within the window. The path of interactionis of dual length L₁ and L₂ differing in path length by ΔL.

The phase changes Δφ_(l) and Δφ_(r) experienced by the left and rightparts of the modal field carried in the path of interaction of duallength L_(l) and L_(r) are different since the path length differs. Thedifference Δ between the phase changes is a much slower function of Δβsince it is determined by the difference in path length:Δ=Δφ_(l)−Δφ_(r) =ΔβΔL=Δβ(L _(l) −L _(r))

If the condition Δ<2π is achieved by designing a suitably smalldifference in path length ΔL, the interference pattern produced from theleft and right parts of the modal field will remain within one cycle ofrepetition (when compared together) through the many cycles of phasechange experienced individually by the left and right parts of the modalfield (see FIG. 3).

By way of example, a stimulus of interest typically causes a change inthe propagation constant (Δβ) of 11.283×10³m⁻¹ (such a change occurswhen water is introduced to replace air as the medium above the surfaceof the sensing waveguide). The maximum difference in path lengthrequired is then given by ΔL=2π/Δβ=0.557 mm.

Aliasing is prevented since the relative phase positions of the parallelparts of the modal field carried in the path of interaction of duallength can be determined before and after the stimulus of interest isintroduced. There is no need to monitor continuously the phase positionof each individual component during the application of the stimulus ofinterest. Obtaining Δ in this way and knowing ΔL leads directly to thedesired Δβ.

Calibration

In place of the absorbent sensing layer 4 f in FIG. 1, it is possible toincorporate a capping layer (or spacer) in which a window of preciselydefined dimensions serves to expose the silicon oxynitride layer 4 e(the sensing waveguide) to a stimulus of interest. For example, acapping layer 4 g may be used to define a dual path of interaction asdescribed above with reference to FIG. 2. The composition and thereforethe refractive index of the capping layer 4 g may be preciselydetermined using standard material deposition methods familiar insemiconductor processing (eg silicon dioxide deposition using PECVD).Photolithography used to define the window typically has a resolution ofless than 1 micron.

Calibration requires the absolute value of the propagation constant ofthe modal field of the sensing waveguide to be determined prior to theintroduction of a stimulus of interest. Provided that the refractiveindex of the capping layer is known, it is possible to performcalibration as follows.

With reference to FIG. 4, the measured difference Δ in output phaseposition of the two parallel parts of the modal field φ_(l) and φ_(r)carried in the path of interaction of dual length is formulated asfollows:Δ=(β_(c)−β_(s))ΔL=ΔβΔL   (1)(where β_(c) is the propagation constant of the part of the modal fieldof the sensing waveguide beneath the capping layer and β_(s) is thepropagation constant of the part of the modal field of the sensingwaveguide in the window.

Provided that the capping layer is sufficiently thick, the difference inthe propagation constants (Δβ) depends only on the difference inrefractive index between the medium in the window and the capping layer.The former may be air for example (whose refractive index is known to beapproximately unity).

Using two orthogonal polarisations (TE and TM), two different values forΔ may be measured (Δ_(TE) and Δ_(TM)) . Equation 1 may be solvednumerically for values of the refractive index and the thickness of thesensing waveguide that provide Δβ_(TE) and Δβ_(TM) corresponding toΔ_(TE) and Δ_(TM). Plots of Δβ_(TE) and Δβ_(TM) versus thickness andrefractive index cross each other and have a common value at a uniquepair of parameters (thickness and refractive index) for the sensingwaveguide. The absolute values for the propagation constants can beobtained by methods known to those skilled in the art.

FIG. 5 illustrates in exposed plan view an embodiment of the integratedoptical waveguide interferometer of the invention. It is largely thesame as that described above with reference to FIG. 1 but the absorbentsensing layer 4 f has been replaced with a capping layer to expose thesurface of the silicon oxynitride layer 4 e acting as the sensingwaveguide. The sensing waveguide mode is carried within a path ofinteraction of variable optical length provided by a gradient X. Thispermits fine details of the stepwise change in phase between the pathsL_(l) and L_(r) to be measured.

FIG. 6 illustrates an embodiment of an integrated optical waveguideinterferometer of the invention. It is largely the same as thatdescribed above with reference to FIG. 1 but without the absorbentsensing layer 4 f. The silicon oxynitride layer 4 e (the sensingwaveguide) is of dual thickness T₁ and T₂ to give a path of interactionof dual optical length as described below.

FIG. 7 illustrates an embodiment of an integrated optical waveguideinterferometer of the invention. It is largely the same as thatdescribed above with reference to FIG. 1 but without the absorbentsensing layer 4 f. The silicon oxynitride layer 4 e (the sensingwaveguide) is of dual composition (ie dual intrinsic refractive indexRI¹ and RI²) to give a path of interaction of dual optical length asdescribed below.

With reference to FIGS. 6 and 7, the optical path length (L′) is relatedto the geometrical path length (L) byL′=λL/n(where λ is the wavelength of the excitation radiation and n is therefractive index of the path of interaction)

Any variation in L′ can be used to determine the absolute phasecondition. Variations in L have been described with reference to FIGS. 2and 5. A sensing waveguide with known variation in refractive index(either dimensional—see FIG. 6 or intrinsic—see FIG. 7) may be usedanalogously.

1. An integrated optical waveguide interferometer capable of detectingthe amount of or changes in a stimulus of interest comprising: a sensingwaveguide capable of exhibiting a measurable response to a change in alocalised environment caused by the introduction of or changes in thestimulus of interest, said sensing waveguide having a path ofinteraction of variable optical length.
 2. An integrated opticalwaveguide interferometer as claimed in claim 1 further including: one ormore sensing layers capable of inducing in the sensing waveguide ameasurable response to a change in the localised environment caused bythe introduction of or changes in the stimulus of interest.
 3. Anintegrated optical waveguide interferometer as claimed in claim 2further comprising: an inactive waveguide in which the sensing layer issubstantially incapable of inducing a measurable response to a change inthe localised environment caused by the introduction of or changes inthe stimulus of interest.
 4. An integrated optical waveguideinterferometer as claimed in claim 1 wherein the variation in opticallength of the path of interaction is sufficient to ensure a variation inphase change caused by the introduction of or changes in the stimulus ofinterest of <2π.
 5. An integrated optical waveguide interferometer asclaimed in claim 1 wherein the path of interaction is stepped.
 6. Anintegrated optical waveguide interferometer as claimed in claim 1wherein the path of interaction is of dual optical length.
 7. Anintegrated optical waveguide interferometer as claimed in claim 6wherein the difference in dual optical length is sufficient to ensure adifference in phase change caused by the introduction of or changes inthe stimulus of interest of <2π.
 8. An integrated optical waveguideinterferometer as claimed in claim 1 wherein the variation in opticallength of the path of interaction is provided by a variation in itsgeometrical length.
 9. An integrated optical waveguide interferometer asclaimed in claim 8 wherein the variation in geometrical length issufficient to ensure a variation in phase change caused by theintroduction of or changes in the stimulus of interest of <2π.
 10. Anintegrated optical waveguide interferometer as claimed in claim 8wherein the geometrical length of the path of interaction is stepped.11. An integrated optical waveguide interferometer as claimed in claim 8wherein the path of interaction is of dual geometrical length.
 12. Anintegrated optical waveguide interferometer as claimed in claim 11wherein the difference in dual geometrical length is sufficient toensure a difference in phase change caused by the introduction of orchanges in the stimulus of interest of <2π.
 13. An integrated opticalwaveguide interferometer as claimed in claim 8 wherein the variation inthe geometrical length of the path of interaction is continuous.
 14. Anintegrated optical waveguide interferometer as claimed in claim 13wherein the path of interaction has a gradient.
 15. An integratedoptical waveguide interferometer as claimed in claim 1 wherein thevariable optical length of the path of interaction is provided by avariation in its refractive index.
 16. An integrated optical waveguideinterferometer as claimed in claim 15 wherein the refractive index isvaried intrinsically by varying the composition of the material of thesensing waveguide.
 17. An integrated optical waveguide interferometer asclaimed in claim 16 wherein the sensing waveguide is composed of two ormore discrete portions of material of differing composition.
 18. Anintegrated optical waveguide interferometer as claimed in claim 16wherein the sensing waveguide is of dual composition.
 19. An integratedoptical waveguide interferometer as claimed in claim 15 wherein therefractive index is varied dimensionally.
 20. An integrated opticalwaveguide interferometer as claimed in claim 19 wherein the refractiveindex is varied dimensionally by varying the thickness of the sensingwaveguide.
 21. An integrated optical waveguide interferometer as claimedin claim 19 wherein the sensing waveguide is of dual thickness.
 22. Anintegrated optical waveguide interferometer as claimed in claim 1further comprising a capping layer adapted to define the path ofinteraction of variable optical length.
 23. An integrated opticalwaveguide interferometer as claimed in claim 22 wherein the cappinglayer incorporates a window which bounds the localised environment. 24.An integrated optical waveguide interferometer as claimed in claim 23wherein the window bounds a medium so that the capping layer defines apath of interaction of at least dual optical length in which a firstpart of a modal field interacts with the medium in the window and asecond part of the modal field interacts with the medium of the cappinglayer.
 25. A process for determining the absolute status of anintegrated optical waveguide interferometer, said process comprising:(A) providing an integrated optical waveguide interferometer thatincludes a sensing waveguide capable of exhibiting a measurable responseto a change in a localised environment caused by the introduction of orchanges in a stimulus of interest, said sensing waveguide having a pathof interaction of variable optical length; (B) irradiating theintegrated optical waveguide interferometer with electromagneticradiation; (C) introducing to the localised environment the stimulus ofinterest; (D) measuring the variation in phase shift of the modal fieldinteracting with the path of interaction; and (E) calculating from thevariation in phase shift the absolute status of the integrated opticalwaveguide interferometer.
 26. A process as claimed in claim 25 whereinthe variation in phase shift caused by the introduction of or changes ina stimulus of interest is <2π.
 27. A process as claimed in claim 25wherein the path of interaction is of dual optical length.
 28. A processas claimed in claim 27 wherein the difference in phase shift caused bythe introduction of or changes in a stimulus of interest is <2π.
 29. Aprocess as claimed in claim 25 further comprising: (F) relating theabsolute status to the amount of or changes in the chemical stimulus ofinterest.
 30. A method for determining the absolute calibration statusof an integrated optical waveguide interferometer, said methodcomprising: (A) providing the integrated optical waveguideinterferometer that includes a sensing waveguide capable of exhibiting ameasurable response to a change in a localised environment caused by theintroduction of or changes in a stimulus of interest, said sensingwaveguide having a path of interaction of variable optical length; (B)irradiating the integrated optical waveguide interferometer withelectromagnetic radiation; (C) measuring the variation in phase positionof the modal field interacting with the path of interaction; and (D)calculating from the variation in phase position the absolutecalibration status of the integrated optical waveguide interferometer.31. A method as claimed in claim 30 wherein the path of interaction isof dual optical length and wherein step (C) includes measuring thedifference in phase position of the first and second parts of the modalfield interacting with the path of interaction of dual optical lengthrespectively.
 32. A method as claimed in claim 30 wherein steps (A) to(C) are performed at start-up.